MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are used for switching power supplies operating at high current and high breakdown voltage. Furthermore, the demand for the MOSFETs has been rapidly increasing in the market for energy-saving switching power supplies used in, for example, mobile communication devices such as notebook personal computers. The MOSFETs are used in a power management circuit and a safety circuit for a lithium-ion battery. The MOSFETs are designed so as to have at lower driving voltage, lower on-resistance and higher breakdown voltage so that it can be directly driven by battery voltage.
These devices are desired to switch using inductive load in consideration of motor control devices. Avalanche breakdown may occur in the MOSFETs, when a surge voltage generated by an induced electromotive force exceeds the breakdown voltage of the MOSFETs during switch-off. For example, it is known to solve this problem that an active clamp circuit which has a diode connected between a gate and a drain with higher breakdown voltage than that of the MOSFETs. If drain voltage of the MOSFETs increases over breakdown voltage of the diode, the diode is broken down before the avalanche breakdown in the MOSFETs occurs during switch-off. This breakdown increases a gate voltage of the MOSFETs and forms a channel.